1. Field of the Invention
The invention relates to a device for the transmission of high-power laser radiation.
Among others, the laser radiation emitted by Nd:YAG lasers in the optical wavelength range is used for machining materials with high-power laser radiation, for example for cutting and welding. One of the advantages gained from the use of a Nd:YAG laser lies in the fact that the laser beam can be transmitted to the work piece by an optical fiber.
Damage to the optical fiber, for example a rupture caused by an excessive or frequently repeated bending stress, poses a danger that a portion of the laser beam escapes or is diverted from the optical fiber into the environment, where it causes damage. It is therefore important to continuously monitor the condition of the optical fiber and to shut off or to divert the laser beam immediately upon detecting a fault. In other words, the beam must be prevented from entering into the optical fiber if a fault is detected.
2. Description of the Related Art
A plurality of devices and procedures are known to the art which are based on optics and serve the purpose of monitoring the condition of an optical fiber disposed in an optical transmission path. From WO 91/17015, for example, a device is known in which, in addition to the high-energy laser light, a light with a different wavelength is coupled into the optical fiber. That light is reflected at the end of the transmission length and received by a receiver device located near the laser. In the case of a rupture of the optical fiber, the intensity of the reflected light received by the receiver device is diminished and leads to a shutoff or dimming of the energy-rich laser light. That device, however, is relatively costly and cost intensive because of the required additional optical components.
It is also known to monitor the condition of an optical fiber by equipping the peripheral surface thereof with a metal wire inserted between the optical fiber and a cable cladding placed over the optical fiber. This is based on the premise that, if the optical fiber ruptures, then the metal wire will break as well. The breaking of the metal wire can be detected via electrical resistance measurement. It has become evident, however, that a partial or complete rupture of the optical fiber does not in all cases lead to the breakage of the metal wire. Monitoring of the electrical resistance of the metal wire, therefore, does not offer reliable information with regard to the condition of the optical fiber.
Optical fibers for optical information transmission are known from German published, non-prosecuted application DE-A1-25 27 769. A metal coating is applied directly onto the glass fiber. The metal coating is applied directly after the manufacture of the glass fiber and simultaneously serves as protective layer for the glass fiber in the later application of a synthetic cladding. The metal coating serves predominantly as protection of the glass fiber against humidity. Another advantage mentioned in the context of that prior art optical fiber is the fact that the metal coating also makes an electrical measurement of a fault location in the optical fiber possible.
Optical waveguide fibers in which a metallic coating is applied directly onto the glass cladding for protecting the interior glass fiber, are also known, for example, from U.S. Pat. Nos. 3,083,550 and 4,893,895, and from German patent applications Nos. 2 556 786; 2 735 079; 2 704 821; and 2 826 010. The art has also proposed to use the metallic protective coating in the transmission of useful electrical signals. These prior art optical fibers thus are well suited for monitoring the glass fiber for ruptures due to their metallic coating, but particularly with applications in which the optical fiber is subjected to constantly changing bending stresses, i.e. particularly for guiding laser beams to a moving tool head for machining a work piece, it has become clear that the optical fibers coated with a metallic material are clearly less resistant to rupture than the optical fibers known, for example, from U.S. Pat. No. 5,015,068, which do not contain a metallic coating between the glass fiber and a synthetic protective layer.
The prior art optical fibers with metallic coating are, therefore, well suited for monitoring for rupture, but their use is only possible with cables in fixed locations which are not subjected to constantly changing bending stresses.
It is accordingly an object of the invention to provide an optical fiber the condition of which can be easily and securely monitored and which ensures long service life even with frequent bending stress.